With present technology laser printing systems, it is common practice to pass the printed media or paper with the just printed text or graphics between a fuser roller and idler pressure roller to burn in or fuse in the text or graphics on the media. This eliminates the possibility of smearing the media thus enhancing the permanent nature of the generated document. As the media leaves the surface of the photoconductive drum and transfer rollers, it is guided into the fuser. In the fuser the media is compressed between a pressure roller and a fuser roller. The fuser roller is heated to a temperature of about 180 degrees Celsius. At this temperature, the toner liquefies thereby fusing to the media. To insure high print quality, the toner must be evenly fused to the paper.
To allow for machining tolerances and assembly errors in the fuser roller itself, the pressure roller has generally been made of a compliant material. Because the pressure roller is made of such material it tends to deform as it presses against the fuser roller. As the paper passes between the fuser roller and a pressure roller, a fixed amount of the media is held in contact with the fuser roller. This point of contact is also known in the art as nip.
While the compliant pressure roller may reduce manufacturing tolerances in the fuser roller itself, it also tends to widen the nip width on the printed media. Additionally, because the clamping and loading forces between the fuser roller and the pressure roller are applied at the ends, the nip width varies across the length. If a completely non-compliant pressure roller is used, the nip width can be reduced to a minimum and kept even. However, with this arrangement the necessary manufacturing tolerance on the fuser assembly makes the system un-manufacturable in mass quantities. If the tolerances are not accounted for, the pressure seen by the media may vary significantly both longitudinally and laterally along the media adversely affecting the print quality.
As the nip width increases, the amount of the print media in contact with the hot fuser roller increases. This translates into a proportional amount of the print media being subjected to the curvature of the fuser roller. Thus the print media is simultaneously exposed to high temperature and curve. As the print media and toner cool, the media is susceptible to retaining the contour of the fuser roller. The net result of this operation is the production of an undesirable curl in the print media. This curl may then hinder subsequent paper handling devices such as photocopying, facsimile machines or the like.
If the nip width is not maintained across the media and down the media, the media tends to deform in a different manner. This uneven nip width, which is generally caused by a varying pressure between the pressure roller and the fuser roller, introduces a characteristic wave in the print media. Wave differs from curl in that wave generally is more of an oscillating type distortion. However, wave can also hinder subsequent paper handling devices. Additionally, a wave gives a document stack a less than professional appearance.